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NBS

PUBLICATIONS

NBSIR 82-2621

Thermal Comfort Conditions in

the NBS/DoE Direct Gain

Passive Solar Test Facility

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Washington, DC 20234

December 1982

Prepared for Passive and Hybrid Solar Energy Division

Office of Solar Heat Technologies

rQQ^^Mwpepartment of Energy

IQQ lington, DC 20545

, U56

62-2621

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NBSIR 82-2621

93 ' THERMAL COMFORT CONDITIONS IN

THE NBS/DOE DIRECT GAIN

PASSIVE SOLAR TEST FACILITY

Stanley T. Liu

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Washington, DC 20234

December 1 982

Prepared for Passive and Hybrid Solar Energy Division

Office of Solar Heat Technologies

U.S. Department of Energy

Washington, DC 20545

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS. Ernest Ambler. Director

FOREWORD

This report is one of a series documenting NBS research and analysis efforts

to support the Department of Energy Passive and Hybrid Solar Energy Program.

The work reported here was sponsored by the Passive and Hybrid Solar Energy

Division, Office of Solar Heat Technologies, U.S. Department of Energy,

Washington, DC, under Interagency Agreement No. EA-77-A-01-6010

.

iii
ABSTRACT

The thermal comfort conditions in a direct gain cell of the NBS/DoE passive

solar test facility were analyzed in accordance with the criteria specified in

the recently revised ASHRAE Comfort Standard 55-1981, using test data collected

during the month of October 1981 and the month of January 1982. It was found

that the daytime operative temperature (as measured by the black globe temperature sensors) in an area near the large south glazing exceeded the upper

boundary of the ASHRAE comfort envelope by a large amount on a clear day during

both the thermal transition month of October and the cold winter month of

January. The generally accepted method of computing the mean radiant

temperature based only on the interior surface temperatures was found to

produce large errors. The reflected solar radiation from the interior surfaces

and the snow covered ground was believed to play a significant role on the

measured black globe temperature and should be included in the computation of

the mean radiant temperature for a space with large glazed areas.

Key words: ASHRAE Comfort Standard 55-1981; black globe temperature; comfort

envelope; direct gain room; operative temperature; passive solar

test facility; solar radiation; thermal comfort.

iv
TABLE OF CONTENTS

Page

FOREWORD iii

ABSTRACT iv

LIST OF FIGURES vi

1. INTRODUCTION 1

2. ASHRAE STANDARD 55-1981 3

3. DESCRIPTION OF THE TEST FACILITY 5

4. INSTRUMENTATION 7

5. TEST CONDITIONS 8

6. TEST RESULTS AND DISCUSSIONS 9

6.1 SOLAR RADIATION DATA 9

6.2 ROOM AIR TEMPERATURE AND THERMAL COMFORT CONDITION 9

6.3 BLACK GLOBE TEMPERATURE AND THERMAL COMFORT CONDITION 9

6.4 COMPARISON OF COMPUTED OPERATIVE TEMPERATURE AND BLACK

GLOBE TEMPERATURE 11

6.5 FLOOR SURFACE TEMPERATURE 12

6.6 EFFECTS OF NONTYPICAL CONDITIONS 13

7. CONCLUSIONS 15

8. ACKNOWLEDGMENTS 16

REFERENCES 17

APPENDIX ESTIMATION OF THE INNER SURFACE TEMPERATURE OF A DOUBLE-PANE

GLASS WINDOW A-l

v
LIST OF FIGURES

Page

Figure 1. The NBS passive solar test building 18

Figure 2. Floor plan of the NBS passive solar test building 19

Figure 3. Sensor location - room and globe temperature along the

center line, and surface temperature on east and west

walls 20

Figure 4. Sensor location - ceiling and floor surface temperature 21

Figure 5. Solar radiation on a horizontal surface during the

October 1981 test 22

Figure 6. Solar radiation on a horizontal surface during the

January 1982 test 23

Figure 7. Room and ambient air temperature during the October test .... 24

Figure 8. Room and ambient air temperature during the January test .... 25

Figure 9. Black globe temperature along the center line at distance

L from south glazing and 1.52 m above the floor during

the October test 26

Figure 10. Black globe temperature along the center line at distance

L from south glazing and 1.52 m above the floor during

the January test 27

Figure 11. Comparison of air and black globe temperature at center

of room during the October test 28

Figure 12. Comparison of air and black globe temperature at center

of room during the January test 29

Figure 13. Black globe temperature along the center line at distance

L from south glazing at 1.52 m above the floor during

October 19-22, 1981 30

Figure 14. Black globe temperature along the center line at distance

L from south glazing at 1.52 m above the floor during

January 17-20, 1982 31

Figure 15. Average hourly surface temperatures near the south glazing

during October 21, 1981 32

Figure 16. Average hourly surface temperatures near the south glazing

during January 17, 1982 33

vi
LIST OF FIGURES (Continued)

Page

Figure 17. Comparison of measured and estimated operative temperature

near the south glazing during October 21, 1981 34

Figure 18. Comparison of measured and estimated operative temperature

near the south glazing during January 17 , 1982 35

Figure 19. Hourly solar radiation data during October 21, 1981 36

Figure 20. Hourly solar radiation data during January 17 , 1982 37

Figure 21. Floor surface temperature variations along the center line

at a distance L from the south glazing during the October

test 38

Figure 22. Floor surface temperature variations along the center line

at a distance L from the south glazing during the January

test 39

vii

1 . INTRODUCTION

* t

Over the past several years, the national concern with the increasing cost of

conventional energy has stimulated the utilization of solar energy for the

heating and cooling of buildings. Specifically, the design of residential

buildings with passive solar heating features has received increasing attention

from both the U.S. Department of Energy (DoE) and the general public because

of the potential for large energy savings at lower cost than for many active

solar heating systems. In a passive system, solar energy which impinges on

the building is collected and stored by the architectural elements of the

building such as heavy-mass walls, floor, or roof, and is distributed by

natural means such as conduction, convection, and radiation without the use of

solar collectors and conventional mechanical equipment. However, the acceptance and growth in the number of passive solar heated buildings will depend

to a large extent on occupant comfort in such buildings. The American Society

of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) Standard

55-1981, "Thermal Environmental Conditions for Human Occupancy," [1], which

defines the condition for thermal comfort in a building, has recently been

revised to include adjustments for extending the comfort zone due to the effect

of air movement. The revisions also set limits on the amounts of temperature

drift within the occupied zones, and on the non-uniformity of vertical temperature distribution, radiant asymmetry and floor temperature. All these have a direct bearing on the determination of the conditions of thermal comfort

existing in passive solar buildings.

This report describes an investigation of thermal comfort condition in a direct

gain type room which is one of the popular generic type configurations in

passive solar heated buildings. In the direct gain configuration, the sun’s

rays are allowed to enter the building's living space directly through a large

expanse of south facing glass arrangement (windows, clerestory, or sliding glass

door). The surfaces and masses of the floor, walls, or ceilings, which are in

direct contact with the building's interior living space, are used as the

thermal collecting, transfer, and storage media for the entering solar energy.

This direct and larger than normal amount of solar radiation into the living

space causes a higher than usual surface temperature on the storage walls and

floor, and a larger radiant energy flux passing through the space due to

reflections and surface emissions. Both the higher surface temperatures anc

the larger radiant flux may cause thermal discomfort for the occupants and are

two of the criteria established in the ASHRAE Standard 55-1981 for the

evaluation of thermal comfort.

The building selected for the comfort analysis in this report is the direct gain

cell of the NBS/DoE passive solar research test facility [2]. The test facility

is one of the DoE Class A passive research facilities constructed in support of

the DoE Program Area Plan [3, 4]. The facility has been in operation since

October 1981, and several preliminary tests, each of from two to three weeks

duration, have been conducted over the fall and winter seasons. The direct

gain cell was extensively instrumented for thermal performance data collection

during those tests. The collected data from two of the preliminary tests, one in the thermal transition month of October 1981, and one in the prime heating

month of January 1982 were analyzed and discussed on the basis of the criteria

1
contained in the ASHRAE Standard 55-1981. A brief description of the ASHRAE

Standard, the direct gain room in the test facility, the test configurations

and indoor physical environment settings, and the evaluation of the thermal

comfort conditions, are presented in the following sections.

2
2.

ASHRAE STANDARD 55-1981

The standard specifying the thermal environmental conditions in a building foi- the comfort of healthy people is ASHRAE Standard 55-1981, "Thermal Environments

Conditions for Human Occupancy” [1], revised in 1981. It states in part that

at the center of the room and 0.6 m (2 ft) from the center of each exposed

wall, the following conditions should be met at all times:

1. The operative temperature, T0 , measured within the occupied zone shall be on the boundary or within the "comfort envelope". The operative temperature is defined as the uniform temperature of a radiantly black enclosure

in which an occupant would exchange the same amount of heat by radiation

plus convection as in the actual nonuniform environment. The comfort

envelope is defined as a quadrangle with the following corner coordinates

when plotted on the psychrometric chart:

Winter: T0 = 19.5-23°C (67 . 1-73. 4°F) at 16.7°C

(62°F) dew-point temperature (DP), and

T0

= 20.2-24.6°C (68 .4-76 .4°F) at 1.7°C

(35°F) DP.

Summer: T0 = 22.6-26°C (72.7-78.8°F) at 16.7°C DP

and

T0 = 23.3-27.2°C (74-80. 9°F) at 1.7°C DP

2. The humidity as described in terms of dew-point temperature shall not

be less than 1.7°C (35°F) or greater than 16.7°C (62°F).

3. The average air movement in the occupied zone shall not exceed 0.15 m/

s

(30 fpm) in winter and 0.25 m/ s (50 fpm) in summer. However, the comfort

zone can be extended above 26 °C (79°F) if the average air movement is

increased 0.275 m/s for each °C (30 fpm for each °F) of increased temperature to a maximum temperature of 28°C (82.5°F) and 0.8 m/s (160 fpm).

4. When the mean radiant temperature (defined as the uniform surface

temperature of a radiant black enclosure in which an occupant would

exchange the same amount of radiant heat as in the actual nonuniforra space)

differs from the air temperature in the occupied zone, the air temperature

shall be adjusted to keep the operative temperature within the appropriate

comfort zone. For low air movement, T0

is approximately the average of the

air temperature and the mean radiant temperature.

5. Monotonic, steady, non-cyclical temperature changes (drifts) shall not

extend beyond the comfort zone by more than 0.6°C (1°F) and for longer than

one hour.

6. The radiant temperature asymmetry for a plane element 0.6 m (2 ft) above

the floor, shall be less than 5°C (9°F) in the vertical direction and less

than 10°C (18°F) in the horizontal direction.

3
7. The surface temperature of the floor for people wearing appropriate indoor

footwear shall be between 18°C (65°F) and 29°C (84°F).

The above criteria are specified for persons clothed in typical summer or winter

clothing, at light, mainly sedentary, activity. The insulating value of clothing

is defined in units of Clo, where 1 Clo = 0.155 m^.K/W (0.88 f t^ »h«F/Btu) . An

example of typical winter clothing is given in ASHRAE 55-1981 as consisting of

heavy slacks, long sleeved shirt and sweater, with an overall insulating value

of 0.9 Clo, and for summer clothing, light slacks and short sleeve shirt (0.5

Clo). In addition to the above criteria, ASHRAE 55-1981 also states the conditions for temperature cycling, nonuniformity in vertical air temperature, for

sedentary but nontypical clothing, and for active persons. Details of these

are contained in Reference [1].

For the present analysis, the occupants were assumed to be in typical winter or summer clothing, and at sedentary or light activity level. Since the tests were conducted during the low humidity and colder months of October and January,

and no humidifier was installed in the cell, the comfort limits of the comfort

envelope under the low dew-point temperature (1.7°C) setting will be applied in

the analysis in this report. Namely, at a dew-point temperature of 1.7°C

(35°F)

,

Winter (0.9 Clo): T0

= 20.2 - 24.6°C (68.4 - 76.4°F)

Summer (0.5 Clo): T0

= 23.3 - 27.2°C (74-80. 9°F).

4
3. DESCRIPTION OF THE TEST FACILITY

The NBS/DoE passive test [2] facility is located at Gaithersburg, Md. (latitude

39°N, longitude 77.3°W) less than one mile from the main NBS campus, on an open

field with no shading from the surroundings. The building is a one-stony , tlabon-grade, rectangular, frame structure with the long axis running from east to

west. Figure 1 shows the south and east elevations of the building. The

building is divided into four re-configurable test cells of equal areas. A

floor plan of the test cells is shown in figure 2. Each cell has a floor area

of 30.1 m^ (342 ft^) and contains a different south facing passive solar feature.

South facing vertical clerestory windows are common to all the cells. The roof

is divided into two sections; a 4.5 ra (15 ft) south section of flat built-up

construction and a 4 m (13 ft) north section of asphalt shingle covered, sloped

roof to accomodate the clerestory windows. The ceiling height under the flat

roof is 2.46 m (8.08 ft). It varies from 4.64 m (15.25 ft) at the south to

2.46 m (8.08 ft) at the north under the sloped roof section. The total volume

of each of the cells is 88.4 m^ (3122 ft^). Both sections of the roof are insulated with 240 mm (9.5 in) of fiberglass batt insulation. The slab-ongrade floor consists of a 100 mm (4 in) concrete slab over 100 mm of gravel on compacted soil. The floor is insulated with 50 mm (2 in) extruded polystyrene

insulation board along the perimeter of the slab down to the footings. The

north outside wall is of singly-studded (2 x 4's) frame construction covered

with insulating sheathing and cedar sidings and insulated with 90 mm (3.5 in)

fiberglass batt insulation. The east and west outside walls are of doublystudded (two rows of 2 x 4's separated by air space) frame construction covered

with insulating sheathing and cedar sidings and insulated with two layers of 90 mm (3.5 in) fiberglass batt insulation. The double-studding and extra insulation are used to reduce the heat transfer through the end walls. The adjacent

cells are separated by heavily insulated partition walls (38 mm (1.50 in)

extruded polystyrene plus 90 mm fiberglass batt insulation). A heavy metal

door insulated with 38 mm (1.50 in) of styrofoam serves as passage way between

the cells. Besides the special solar features described later, the glazed

areas common to all four cells consist of a double-pane, wood casement window

with an unobstructed glass area of 0.67 m^ (7.2 ft^) located on the corner of

the north wall of each cell, and a clerestory window located on the upper level

of the south wall. The clerestory window consists of a three-section, doublepane, wood awning window which spans the entire width of the cell with an unobstructed glass area of 3.65 m^ (39.25 ft^). Both the north window and the

clerestory window are provided with an operable interior insulating shutter

with a thermal resistance value of 1.23 m^.R/W (R=7).

A different passive solar feature is incorporated into each of the cells. The

direct gain cell (cell 4 in figure 2) studied in this report contains a double

pane wood sliding glass door covering the entire width of the lower level south

facing wall. The sliding glass door consists of two fixed sections and one sliding section. All three sections are of equal areas. The total unobstructed

glass area is 6.27 m^ (67.53 ft^).

Besides the thermal storage provided by the concrete floor, this cell contains

an additional thermal storage in the form of a 2.08 m wide by 2.46 m high

(6.83 ft by 8.08 ft) concrete block wall located inside the north wall. The

5
block wall is made of 200 mm by 400 mm by 200 mm (8 in by 16 in by 8 in) concrete

masonry unit (CMU) with cores filled with mortar. The surface of the block is

unpainted. A detailed description of this cell and the other three cells is

given in [2].

6
4. INSTRUMENTATION

Extensive instrumentation was installed in 1981 in the direct gain cell as an initial step in instrumenting the entire test facility. The measured data

included the following;

£ Solar radiation: total horizontal, total south facing vertical inside and

outside of the sliding glass door, normal beam radiation, and infrared sky

radiation.

£ Weather: wind speed and direction, barometric pressure, ambient air

temperature.

£ Relative humidity: inside and outside.

£ Temperature: globe temperature, inside air temperatures, temperatures on

all inside surfaces including the floor and ceiling, ground temperatures,

and foundations wall temperatures.

£ Heat fluxes: at the ceiling, east wall, north storage wall, and floor.

£ Air infiltration.

£ Energy consumption: lights and equipment, auxiliary heating, and cooling.

Figures 3 and 4 show the locations of the relevant air and surface temperature

thermocouples, and black globe sensors. A detailed description of all the sensors and their locations is given in [2].

With the exception of the air infiltration data, which is a one-time

measurement, and the energy consumption data which are measured separately,

all other data are taken by an on-site data acquisition system and recorded

onto magnetic tape at ten-minute intervals. The ten-minute interval data are numerically integrated and averaged into an hourly data set.
5. TEST CONDITIONS

Several preliminary thermal performance tests of the direct gain cell, each

for a duration of from two to three weeks, have been conducted over the fall

and winter seasons since October 1981. The collected data from two of the

tests, one in the thermal transition month of October 1981 and one in the

prime heating month of January 1981 , were used for the thermal comfort analysis

presented in this report. The October test was a floating temperature test

where the room air temperature was unrestricted (free floating) with no auxiliary cooling or heating. The January test was a fixed temperature test where

the minimum room air temperature was fixed by the heating thermostat setting at

18.3°C (63°F). The air temperature was free to float upward without restriction

since no auxiliary cooling was provided.

During both of these tests, the south facing clerestory window shutter and the

north window shutter were closed. No interior drapery or shading device was provided for the south facing sliding glass door which was kept closed during

the tests. The room air temperature was kept mixed by the action of two small

destratifying fans which were enclosed in two ducts and placed near the center

of the room. The ducts extended from the floor to the sloped ceiling level and were open on both ends. One fan takes the room air from the floor level and

discharges it at the sloped ceiling level near the clerestory window; the other

fan takes the room air from the sloped ceiling level and discharges it at the

floor level. Destratification was also promoted by the blower in the fan-coil

unit below the north window which was turned on and run continuously throughout

the two tests.

8
6. TEST RESULTS AND DISCUSSIONS

Relevant test data from the two test periods were analyzed and the thermal

comfort conditions in the direct gain cell were evaluated on the basis of the

ASHRAE Standard 55-1981 as discussed below. The relevant data for thermal

comfort analysis were the solar radiation, the outside air temperature and

the room air temperature variations, the black globe temperature variations at

three locations in the room, the floor surface temperature variations at four

locations, and the surface temperatures of the interior surfaces.

6.1

SOLAR RADIATION DATA

The solar radiation data on a horizontal surface for an 18-day (432 hour) period

during each of the two tests are shown in figures 5 and 6 for the October test

and the January test, respectively. The test periods contain a mix of clear,

partly clear, and cloudy days. It is worth mentioning that several snow falls

occurred during the January test and the ground around the test facility was covered with snow most of the time. This caused a large increase in the solar

reflectivity of the ground and increased significantly the amount of solar

radiation through the south glass as will be discussed later in this report.

6.2

ROOM AIR TEMPERATURE AND THERMAL COMFORT CONDITION

Figures 7 and 8 show the ambient temperature and room air temperature variations

in the direct gain cell for each of the 18-day (432 hour) test periods in

October 1981 and January 1982, respectively. The horizontal line on figures 7 and 8 are the upper bound (for summer time clothing) and lower bound (for winter

time clothing) of the ASHRAE 55-1981 comfort limits described in section 2 of

this report. It is assumed that the occupants will adjust their clothing when

necessary. It is seen in figure 7 that without thermostat control (no auxiliary

heating or cooling) during the October test, the room air temperature fluctuated

daily between the two limits. This fluctuation is due to the effects of the

solar radiation and higher ambient temperature at daytime, and the absence of

solar radiation and lower ambient temperature at night. The predominant effect

of solar radiation on the daytime room air temperature is evident in figure 8

during the January test. The room air temperature is seen to float upward

from the heating thermostat setting of 18.3°C (65°F) whenever the solar radiation was high, and actually reached the ASHRAE 55-1981 upper limit on three

days during the 18-day test period. Because the room air temperature was well

mixed and controlled at the 18.3°C setting during the January test, the effect

of low ambient and glass surface temperatures on the thermal comfort conditions

during cloudy and snowing days and at nighttime was not evident in the room

air temperature measurement. However, their effect will be shown in the globe

temperature measurement discussed in the following section.

6.3

BLACK GLOBE TEMPERATURE AND THERMAL COMFORT CONDITION

The black globe temperature is measured as the equilibrium temperature at the

center of a 150 mm (6 in) diameter thin wall-sphere with a nonref lective surface having an emissitivity of 0.95. At low air movement condition and in an environment where infrared radiation predominates, it is approximately a measure

9
of the operative temperature and therefore the thermal comfort conditions in an occupied zone [1]. Figures 9 and 10 show the black, globe temperature

variations in the direct gain cell at three locations for each of the 18-day

test periods in October 1981 and January 1982, respectively. Again shown in

the figures are the lower and upper operative temperature limits for comfort

in an occupied zone as specified in ASHRAE 55-1981. The upper limit is for a person in typical summer clothing at light activity level and the lower limit

is for a person in typical indoor winter clothing at light activity level. It

is assumed that the occupants will adjust their clothing when necessary. The

three black globes sensors (shown in figure 3) were located along the south to

north center line of the room, 1.52 m (5 ft) above the floor (in the shade all

the time), and at a distance of 1.52 m (5 ft), 4.72 ra (15.5 ft), and 7 ra (23

ft) from the south glazing, respectively. It is seen from figures 9 and 10

that the black globe temperature 1.52 m from the south glazing (directly above

the sunlit floor and close to the south glazing) frequently exceeded the upper

ASHRAE comfort limit during the daytime hours by from 1°C (1.8°F) to as much

as 7.5°C (13.5°F) and for a period of more than six hours whenever the days

are clear (see figures 5 and 6). The daytime ambient temperature played only

a minor role as can be seen by comparing the results of the mild October days

with the very cold January days. Both the magnitude and the length of time

where the black globe temperatures were over the limit greatly exceeded the

comfort criteria for the temperature drift specified in reference [1] and

described previously in item 5 of section 2 in this report (not greater than

0.b°C over the the limit and not longer than one hour), indicating that over- heating and thermally uncomfortable conditions existed in the occupied space

at least 1.52 m (5 ft) away from the south glazing. This condition occurred

even when the air temperature in the space remained within a comfortable

range most of the time. This uncomfortably hot condition was much less severe

further away from the south glazing as can be seen from data on the other two

black globe temperature sensors. Figures 11 and 12 show a comparison of the room air temperature with the black globe temperature at the middle of the test

cell for the two test periods. It is seen that the black globe temperature and

the air temperature became much closer to each other, as is generally the case

in a normal living space other than a direct gain space.

In figures 9 and 10, it is noted that the lower operative temperature limit,

as well as the criteria for temperature drift of the ASHRAE Comfort standard,

was exceeded frequently, especially during the January test when the weather

was cold. This is caused by the radiation heat loss to the large cold glass

surface and the cold interior wall surfaces during non-solar hours and cloudy

days. However, this cold condition is correctable by employing drapery or movable nighttime insulation over the glazing to reduce the radiation heat

loss and by increasing the setting of the heating thermostat. On the other

hand, the hot condition in an area as much as 1.52 m to 3m (5 ft to 10 ft)

away from the large south glazing will be harder to correct since any covering

of the glazing during the cold January month will defeat the purpose of the

direct gain design. This is not true in October where movable covering in the

daytime when needed, would be very helpful in reducing the overheating condition. Figures 13 and 14 show a replot of the data on figures 9 and 10, but

for a selected period of only four days for easier tracking and identification

of the hourly variation of the black globe temperatures. The four days in

October (19,20,21, 22), figure 13, covered a partly sunny day followed by two

10
clear days and a partly sunny day. The four days in January (17,18,19,20),

figure 14, covered two clear days with a partly sunny and a cloudy day in

between.

6.4 COMPARISON OF COMPUTED OPERATIVE TEMPERATURE AND BLACK GLOBE TEMPERATURE

In an effort to identify the source of the thermally uncomfortable condition

in the region near the south glazing, a computation was made to estimate the

operative temperature at the south black globe temperature sensor location,

using the method described in the ASHRAE Standard 55-1981 [1]. By assuming an average air movement of less than 0.15 m/s (30 f pm) , the operative temperature

at a location can be approximated as the arithmatic average of the air temperature and the mean radiant temperature at that location in the living space.

The mean radiant temperature, Tmrt , can be computed from the inside surface

temperatures and the associated angle factors as given by ASHRAE 55-1981. In

equation form, the mean radiant temperature is written as [1]

Ct - Vl T

l 4 + Fp-2

'C

2 + F T p-n n

( 1 )

where T n

= absolute temperature of surface n,

Fp_n

= angle factor from person at location p to surface n,

n = total number of surfaces.

For the present study, the mean radiant temperature and the operative

temperature for a clear day in October (10-21-81) and a clear day in January

(1-17-82) were computed, using the average temperature data on the interior

surfaces at areas near the south glazing. Since the inside surface temperature

of the glazing was not measured, that temperature was estimated by a simple

quasi-steady state (mass of glass ignored) energy balance, using the measured

solar radiation data on the south vertical surface, the wind speed, and the

inside and outside air temperatures data as input, and an estimated glass (per

layer) solar transmittance of 0.73 and solar reflectance of 0.07. Details of

the energy balance on the glass are given in the appendix of this report.

The results of this investigation are shown in figures 15 to 20.

Figures 15 and 16 show the measured interior surface temperatures, the interior

room and ambient air temperatures, the estimated inside glass surface temperature, and the measured black globe temperature, used in the computation of the

operative temperature for the two selected days. It is noted that most of

the interior surfaces and air temperatures were less than the measured black

globe temperature during the daytime hours. This fact is shown in figures 17

and 18. Figures 17 and 18 show the comparison of the computed and measured (the

black globe temperature) operative temperature at the location 1.52 m (5 ft)

from the south glazing and 1.52 m above the floor. The agreement is very good

at the non-solar hours, but becomes very poor during the high solar radiation

hour with the measured value above the computed value by as much as 3°C (5.3°F)

in October and 8°C (14.4°F) in January. Since the interior surface temperatures

and air temperature were all below the measured black globe temperature, it

11
appears that this difference was caused mainly by the short wave solar radiant

fluxes passing through the interior space where the black globe sensor was located. Parts of those solar fluxes are reflected by the portions of the

floor and interior surfaces near the glazing that receive the solar beam

radiation, and another parts come from the ground reflected solar radiation

that passes through the glazing into the interior space. These radiant fluxes

are not considered in the ASHRAE equation (equation (1)) for the mean radiant

temperature because their effects are considered small for a normal sized window area, however, for the room with a large direct gain type glazing, and

highly reflective surfaces in the solar spectrum, equation (1) should be modified

to include terms for the interior and exterior (ground) reflected solar fluxes.

At this point it should be noted that the black globe sensor measures the

highest possible mean radiant temperature due to its high absorptivity in both

the infrared and the solar spectrum. For surfaces with a lower solar absorptivity such as people in light colored summer clothing, the effect of the

reflected solar fluxes will be less than those measured by the black globe sensors reported here.

In figures 17 and 18, it is noted that the disagreement is much greater in

January than in October. This can be explained by the larger sunlit area on

the interior surfaces caused by the lower altitude angle of the sun, and by

the solar radiation data shown in figures 19 and 20. From figure 19, the

ratio of the insolation on the south vertical surface to that on the horizontal

surface at noon time is 1.24, and the ground reflectivity is 0.2, while from

figure 20, they are 2.03 and 0.55, respectively. The large ground reflectivity

in January was caused by ground snow cover, which, in addition to giving a larger reflected solar radiation, would also cause some specular reflection

(especially when glazed over) of the direct beam radiation through the large

glass into the normally shaded interior space high above the floor level.

This specular reflected beam radiation would have a larger effect on the mean

radiant temperature than the diffuse reflected radiation since its intensity

is not diminished by the distance between the south glazing and a point in the

interior space. This is part of the reason that the black globe temperature

in the middle of the room was higher than the air temperature in a January day

shown in figure 12.

6.5 FLOOR SURFACE TEMPERATURE

Figures 21 and 22 show the surface temperature of the concrete floor at four

locations along the south to north centerline of the direct gain cell during

the October and January tests, respectively. During the month of October,

approximately 25 percent of the floor area received direct solar radiation at

solar noon. Specifically, the portion of the floor measured within 2.2 m

(7.2 ft) from the south glazing in October received direct solar radiation at

solar noon. The effect is shown in figure 21 where the line labeled L = 1.02 m

shows a much larger increase during the day than the rest of the locations.

The temperatures remain essentially uniform for the portions of the floor

which do not see direct sunlight during the day. The nonuniformity in floor

temperature is even more pronounced during the month of January due to the

12
lower altitude angle of the sun in the sky; during this month, the portion of

the floor up to 3.4 m (11.2 ft) from the south glazing is sunlit at solar noon. This effect is shown in figure 22 where the locations on the floor at 2.2 m and

3.1 m from the south glazing show a large difference in temperature from the

rest of the floor area. It is noted that the location closest to the south

glazing (at L = 1.02 m) also recorded the lowest temperature at nighttime and

during cloudy days. This is caused, in addition to the normal perimeter slab

losses, by the radiant heat loss from the floor to the cold glazing surface

during those time periods. Movable nighttime type insulation or drapery should

reduce these heat losses and provide a more comfortable floor temperature during

those non-solar hours.

In figures 21 and 22, the ASHRAE 35-1981 limits on floor surface temperature

(described in section 2 of this report) are also shown. It is seen that the

upper limit was frequently exceeded in the sunlit floor area during the October

test, and the lower limit was exceeded in the same area at nighttime and cloudy

days during the January test. Movable window covering in the daytime in the

thermal transition month of October and in the nighttime in the cold January

month would reduce or eliminate these problems.

6.6 EFFECTS OF NONTYPICAL CONDITIONS

1. Air movement: In the present study, natural circulation was assumed to

exist within the living space where the air movement in the occupied zone

was less than 0.15 m/s (30 fpm). ASHRAE Std. 55-1981 specifies that the

upper boundary of the comfort zone can be extended to above 26°C (79°F)

if the average air movement is increased 0.275 m/s for each 1°C of increased

temperature (30 fpm for each 1°F) to a maximum temperature of 28°C (82.5°F)

and air movement of 0.8 m/s (160 fpm). Therefore, if some mechanical means

were used to increase the air movement 0.8 m/s during high operative temperature periods, the occupied zone would stay within the extended upper

comfort boundary for a longer period of time. However, as is evident from

figures 9 and 10, overheating and uncomfortable condition would still exist

near the south glazing area.

2. Nontypical clothing: ASHRAE Std. 55-1981 specifies that for each 0.1

decrease or increase in Clo value, the comfort zone boundary operative

temperature can be increased or decreased respectively by 0.6°C. With a minimal clothing of 0.05 Clo, the upper boundary can be extended to 29°C

(84°F). Therefore, if the occupants would adjust (reduce) the clothing to

below the typical summer clothing's value of 0.5 Clo during the high

operative temperature period of the day, the length of the time period when

the comfort criteria was exceeded would be reduced or sometimes eliminated.

3. Active persons: The comfort zone boundary values stated in section 2 are

for a sedentary or slightly active person. If the activity level of the

occupants are increased, the standard requires a decrease in the upper

boundary value. For example, for a person doing domestic work (medium

activity) and wearing clothes with a Clo value of 0.35, the reduction in

13
the upper boundary temperature would be by 3.2°C (5.8°F). This would cause

the operative temperature at the center of the room to exceed the allowable

boundary value of 24°C (75.2°F) frequently in both October and January as can be seen in figures 9 and 10.

14
7 . CONCLUSIONS

Based on the study of the preliminary test data collected during the October

1981 and January 1982 tests on the direct gain cell of the NBS/DoE passive test

facility, the following conclusions can be drawn:

1. The occupied zone near the south glazing of a direct gain type space with

large south glazing area will exceed the upper comfort limit specified in

the ASHRAE thermal comfort standard 55-1981 during daytime hours in a clear day during both the thermal transition month of October and the prime

heating month of January. However, mechanical cooling and movable window

covering should reduce or eliminate this condition during October and

presumably also during the summer months.

2. The same region described above would also fall below the lower comfort

limit at nighttime and during cloudy or snowing days in the winter months,

caused by nighttime infrared radiation heat loss to the large cold south

glass surface. Removable insulation or window covering may help in

alleviating this condition.

3. Ground and interior surface reflected solar radiation fluxes near the south

glazing area contribute significantly to the uncomfortably warm condition

near the large south glass area common to a direct gain space. Snow

covered ground in winter, which may reflect specularly, plays a significant

role in increasing the operative temperature of the occupied space.

4. For a direct gain type building with large glass area, the equation

described in the ASHRAE Standard 55-1981 for the estimation of the mean

radiant temperature in an occupied space may need revision to include

terras for the reflected solar radiation fluxes passing through the space

near the large south glass even when the space is shaded by the overhang

roof from the direct solar radiation.

5. Nighttime infrared radiation heat loss from the floor to the large and cold

south glazing caused the floor surface temperature to drop below the ASHRAE

specified limit on floor temperature during the cold winter season. Removable window covering should again be able to alleviating this condition.

It should be noted that the results of this analysis are limited by the

conditions under which the tests were conducted. In order to obtain and provide clean data to be used for the verification of energy analysis computer

programs, those preliminary tests were run without any window covering at any

time during the test. There is no doubt that window covering, even the everyday type, will change the indoor environment to a more comfortable level during

some parts of these tests as discussed above. Additional testing with the

proper and realistic application of window covering as part of the test procedure is therefore recommended for a better evaluation and understanding of the

thermal comfort conditions in a direct gain space. Finally, the role of the

reflected solar radiations from both the interior surfaces and the ground,

especially if the ground is covered with snow, on the room mean radiant

temperature computation should be studied in more detail and quantified.

15
8 . ACKNOWLEDGMENTS

The author wishes to acknowledge the major contribution of Dr. Mark McKinstry

of Bonneville Power Administration/DoE and Mr. Thomas E. Richtmyer of Tyndall

Air Force Base in the design, construction and operation of the test facility

while both were employed at NBS. The work reported here was sponsored by the

Passive and Hybrid Solar Energy Division, Office of Solar Heat Technologies,

U.S. Department of Energy, Washington, D.C. 20545, under Interagency Agreement

No. EA-77A-01-6010.

16
REFERENCES

1. ASHRAE Standard 55-1981, "Thermal Environmental Conditions for Human

Occupancy," ASHRAE, 345 E. 47th Street, New York, N.Y. 10017, 1981.

2. Lindler, K.W. , "National Bureau of Standards Passive Solar Test Building

Handbook," NBS-GER-82-398 , August 1982.

3. "Program Area Plan: - Performance Evaluation of Passive/Hybrid Solar

Heating and Cooling Systems," Solar Energy Research Institute Report

SERI/PR-721788, October 1980.

4. Hamilton, B.L. and Scofield, P.M. , "National Passive/Hybrid Performance

Evaluation Program for System Development," Proceedings of the 5th National

Passive Solar Conference, Amherst, Ma. , October 1980.

17
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APPENDIX - ESTIMATION OF INNER SURFACE TEMPERATURE OF A DOUBLE-PANE GLASS

WINDOW

For the present analysis, both layers of the double-pane glass window are assumed to be thermally mass-less so that quasi-steady state exists at any time

of the day. Namely, it is assumed that for each glass layer, the heat gain by

the absorption of solar radiation and convection is equal to the heat loss by

convection and radiation to the surroundings. In addition, it is assumed that no temperature gradient exists in each glass layer, that is, the layer is at a uniform temperature.

Assuming that the solar radiation on the vertical glass surface H, the wind

speed Ws , the ambient temperature T 0 , the inside room temperature T^, and the

solar properties of the glass are known, an energy balance on the outside glass

layer gives,

a(H + TpH) = h0 (Ti - T0 ) - us (T 2 - T L

) (A-l)

where a = solar absorptivity of the glass layer

t = solar transmissivity of the glass layer

p = solar reflectivity of the glass layer

hQ = heat transfer coefficient of the outside surface

us = thermal conductance of the air space between the two layers of

glass

T^ = temperature of the outside glass layer

T2 = temperature of the inside glass layer

The left hand terms in equation (1) are the portions of the incoming solar

radiation and the reflected solar radiation from the inside layer surface that

are absorbed by the outside glass layer. Any re-reflection is considered small

and ignored. The right hand terms are heat loss to the ambient air and heat

gain from the inside layer through the air space.

Similarly, an energy balance on the inside glass layer gives

<x(tH) = h± (T2 - T ± ) + us (T 2 - Ti) (A-2 )

where h-^ = heat transfer coefficient of the inside surface.

Equations (A-l) and (A-2) are the two equations for the two unknown T^ and T2

. Combining equations (A-l) and (A-2) and solve for T 2 gives

T2 = h

ous To + h

o h

iTi + h

iu

sT

i + otH((1 + T P )us + fOip+iis))

(h

ohi + us ho + ush

i>

(A-3)

A-l
Values of and us are given in the ASHRAE Handbook of Fundamentals. For this

report, the values of h^ and us are

h

± = 8.29 W/m2 »K

u

s = 7.84 W/m2 »K

hQ is a function of the wind speed and is also given by ASHRAE, for smooth

glass surface, as

h

Q = - 0.0355 • Wg

2 + 3.327 • W

g + 8.233 W/m2 .K

where Ws = wind speed in m/s.

A2
NBS-1 14A (REV. 2-BCI U.S. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication Date BIBLIOGRAPHIC DATA

REPORT NO.

SHEET (See instructions) NBSIR 82-2621 (DoE) December 1982

4. TITLE AND SUBTITLE

THERMAL COMFORT CONDITIONS IN THE NBS/DoE DIRECT GAIN PASSIVE

SOLAR TEST FACILITY

5. AUTHOR(S)

Stanley T. Liu

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) 7. Contract/Grant No. NATIONAL BUREAU OF STANDARDS DE-A101-7 6PRO-6010

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

8. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

U.S. Department of Energy

Passive and Hybrid Solar Energy Division

Washington, D.C. 20585

10. SUPPLEMENTARY NOTES

J Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here) The thermal comfort conditions in a direct gain cell of the NBS/DoE passive solar test

facility were analyzed in accordance with the criteria specified in the recently

revised ASHRAE Comfort Standard 55-1981, using test data collected during the month of

October 1981 and the month of January 1982. It was found that the daytime operative

temperature (as measured by the black globe temperature sensors) in an area near the

large south glazing exceeded the upper boundary of the ASHRAE comfort envelope by a large amount in a clear day during both the thermal transition month of October and

the cold winter month of January. The generally accepted method of computing the mean

radiant temperature based only on the interior surface temperatures was found to

produce large errors. The reflected solar radiation from the interior surfaces and

the snow covered ground was believed to play a significant role on the measured black

globe temperature and should be included in the computation of the mean radiant

temperature for a space with large glazed areas.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

ASHRAE Comfort Standard 55-1981; black globe temperature; comfort envelope; direct gain

room; operative temperature; passive solar test facility; solar radiation; thermal

comfort

13. AVAILABILITY 14. NO. OF

PRINTED PAGES

[Xj Unlimited

[33] For Official Distribution. Do Not Release to NTIS

[[[] Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

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[j£j Order From National Technical Information Service (NTIS), Springfield, VA. 22161 $7.50

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